A blog about ecology of the savanna biome and other regions of interest to safari guides and visitors to East Africa.

Wednesday, 25 January 2012

How to survive the Serengeti: predation, food and body size

Serengeti Lions eat a diversity of mammal species.

Another paper describing one of my favourite talks from the TAWIRI conference back in December has just been published, this time in the Journal of Animal Ecology (available to some here). Grant Hopcraft and colleagues are interested in why different grazing animals use different parts of Serengeti in different ways when they all eat grass. Why, for example, do you usually find buffalo around riverine areas, whereas gazelles tend to be on the open plains? At some level it's obvious that where you find an animal is the place that it survives best and there are two aspects to survival that most ecologists would agree are crucially important: where will you eat? And where will you be eaten? Although both buffalo and gazelles eat grass, and both are eaten by large cats, it's quite possible that grazers of different size will be affected differently by these same two processes. And that's what Grant and colleagues set out to test.

Now Grant and his colleagues set about testing this through the known distribution of animals (from areal survey data of buffalo, topi, Coke's hartebeest, Thomson's and Grant's Gazelles) using statistal modelling methods. They (sensibly) decided there might be different factors that explain presence or absence in a particular census square to the factors that determine if there are any animals there at all. (For example, for some animals that drink a lot presence of water within, say, 2km might be essential to presence, but as long as there's a little bit of water, more water won't mean more animals, whilst more food might well.) So they set about modelling in a two stage way - one to predic presence or absence, then another to predict the relative abundance of each species. Having done that they set up a model that isn't often used by ecologists, called a Structural Equation Model, which allowed them to link things they could actually measure in the field (like bush cover, nitrogen content of grass, grass biomass, etc.) into the three main sets of processes that they considered likely to influence abundance: those factors to do with food quantity (like rainfall), those affecting food quality (like nitrogen content) and those affecting predation risk (like presence of bushes), and then set off to see if their predictions held: did smaller grazers really select high quality areas and avoid areas with high predatin risk? Did buffalo just focus on places with lots of grass, irrespective of predation risk and foraging? And where topi and hartebeest somewhere in between? And I guess they were pleased when they found the answers, broadly speaking, were yes.

In particular, buffalo don't care about anything but food abundance, the gazelles were the only things to chose the high nutrious, low biomass areas with low predation risk, and the hartebeest avoided both high biomass and high quality areas, perhaps searching for low quality grazing in areas with low predation risk, though they were the 'wrong' way round relative to topi, given their body mass. But still, body size alone and it's interaction with food quality, quantity and predation risk can explain some of the patterns of distribution in these species. The key here, of course, is that some - and it's a rather small amount of variation that the authors explain. That's not surprising to me - masses of things affect animal distributions, especially in herding species like these. The analysis can't really consider social effects and there's also bound to be some massive random variation in just where the animals are when the surveys happen - try again a day later and you'd have different patterns to explain. So not a bad effort at least! (Anyone who knows my gripes about distribution modelling, however, will know I've one or two issues with some of the statistics, but nothing that will make a huge influence to the overall results, I guess...)

And another question for Grant - why not include wildebeest? Beacuse you knew they wouldn't fit the pattern? Or what?!

Anyway, the summary is that, to survive in the Serengeti, you need to know how big you are. As for me, I'm somewhere between a Grant's Gazelle and a Topi - that means I should definitely be looking to avoid places of high predation risk, and I should probably be looking for my grass to be short and nutritious - if it doesn't taste good, I'd better not bother... Sticking to the short grass plains definitely sounds like a good survival strategy to me!

A note on the blog

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